The invention relates to a system for injecting a liquid into a pipe carrying water or another liquid at a predetermined rate of flow in proportion to the flow in the pipe. It relates particularly to a system adapted to inject liquid fertilizer into a pipe line carrying water for the irrigation of fields.
In addition to pumps driven by an external power source, serving to inject fertilizer into a water supply line, different devices are known wherein a small pressure drop in the line is utilized for this purpose. Among these devices are Venturi-type and double-Venturi type injectors, their main drawback being the difficulty of exact dimensioning the addition of liquid fertilizer which should be proportional to the irrigation water flow through the supply line.
Other known systems for injecting liquid fertilizer into a pipe carrying irrigation water comprise a turbine impeller driven by the irrigation water and itself driving a pump which draws fertilizer out of a container and pumps it into the pipe line, usually at a location downstream of the turbine.
Such a system is described in U.S. Pat. No. 3,570,508 (BOGS) which comprises an axial turbine impeller coaxially positioned in the pipe and a pump in the form of a helical rotor rotating within a flexible stator housing. Valve means are provided for controlling the fertilizer flow.
The main drawback of this system is the low efficiency of the helical pump, more particularly its enclosure in a flexible stator housing causing considerable friction losses.
Another known embodiment of a fertilizer injector comprises a tangential turbine and a pump mounted on a common shaft in a casing positioned outside an irrigation pipe. The turbine is driven by a measured flow of water drawn from the pipe and spilled into the open after having passed the turbine. The pump draws fertilizer out of a container and injects it into the pipe downstream of the port delivering water to the turbine. Two kinds are proposed as alternatives: (1) a spiral-type screw conveyor, (2) a centrifugal impeller, both kinds of pumps being enclosed in a casing which communicates at its low-pressure side with a fertilizer container, and at its high-pressure side with the irrigation pipe. This system suffers from the drawback that the used water is spilled into the open, and that the pump impeller is necessarily of low efficiency.
Another embodiment of a fertilizer injection system comprises an axial-flow turbine impeller coaxially positioned in the irrigation pipe, a gear pump positioned outside the pipe on a mounting plate, and a gear train extending between turbine and gear pump. Again, as in the aforementioned systems, the pump injects liquid fertilizer into the pipe carrying the irrigation water. The main drawback of this embodiment is the requirement of bevel gearing for transmission of power from turbine to pump, which is usually a source of trouble, especially as the gear is submerged in the water in the pipe and is not readily accessible.
All the aforementioned systems comprise a pump sealingly separated and distanced from the turbine, thus requiring a power transmission in the form of a shaft or a gear train. Both the pump and the transmission absorb power by friction in the bearings and in the fluid seals, considerably reducing the power output of the turbine, which should be small in any case in order not to cause a large pressure drop in the irrigation pipe.